Self-scattering on large, porous grains in protoplanetary disks with dust settling

Abstract

Observations of protoplanetary disks (PPDs) at sub-mm wavelengths suggest that polarisation is caused by scattering of thermal radiation. Most of the dust models that are used to explain these observations have major drawbacks: They either use much smaller grains than expected from dust evolution models, or result in lower polarisations than observed. We investigate the effect of dust grain porosity on the polarisation due to scattering at sub-mm wavelengths arising from grains of up to mm sizes, as they are expected to be present in the midplane of PPDs. Using the effective medium theory, we calculated the optical properties of porous dust and used them to predict the behaviour of the scattering polarisation. Subsequently, radiative transfer simulations for PPDs with porous dust were performed to analyse the additional effect of the optical depth structure, and thus the effect of multiple scattering events and inhomogeneous temperature distributions on the polarisation. We find that porous dust with moderate filling factors of about 10\% increase the polarisation compared to compact grains. For higher grain porosities, that is, grains with a filling factor of 1\% or lower, the extinction opacity decreases, as does the optical depth of a disk with constant mass. Consequently, the unpolarised direct radiation dominates the scattered flux, and the polarisation drops rapidly. Even though the simulated polarisation is higher than in the case of compact grains, it is still below the typical observed values for face-on disks. However, the polarisation can be increased when crucial model assumptions derived from disk and dust evolution theories, for instance, dust settling and mm-sized dust grains, are dropped. In the case of inclined disks, however, our reference model is able to achieve polarisation degrees of about 1\%, and using higher disk masses, even higher than this.

Accepted by A&A

References in corpus (8)

• ALMA Reveals Transition of Polarization Pattern with Wavelength in HL Tau's Disk
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• Solving grain size inconsistency between ALMA polarization and VLA continuum in the Ophiuchus IRS 48 protoplanetary disk
• Self-scattering of non-spherical dust grains
• Self-scattering in protoplanetary disks with dust settling
• Evolution of dust porosity through coagulation and shattering in the interstellar medium
• Polarization reversal of scattered thermal dust emission in protoplanetary disks at (sub-)mm wavelengths
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Cited by in corpus (9)

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• Sub-mm/mm optical properties of real protoplanetary matter derived from Rosetta/MIRO observations of comet 67P
• Is the mm/submm dust polarization a robust tracer of the magnetic field topology in protostellar envelopes? A model exploration
• Polarimetric investigation of selected cloud compositions in exoplanetary atmospheres
• Dust mass in protoplanetary disks with porous dust opacities
• Improving Monte Carlo radiative transfer simulations: A shift of framework
• Wavelength-dependent far-infrared polarization of HL Tau observed with SOFIA/HAWC+